The Chicago Manual of Style provides extensive instructions for two different citation systems:
Chicago Notes and Bibliography style (sometimes called "Chicago A") uses footnotes or endnotes supplemented by a concluding bibliography. This style is commonly used by scholars in the fields of history and the humanities. This format is discussed at length in chapter 14 of the Manual.
Chicago Author/Date style (sometimes called "Chicago B") uses parenthetical or in-text references containing author last name and date of publication, with full bibliographical detail given in a concluding reference list. This style is used more often in the sciences, and is discussed at length in chapter 15 of the Manual.
Kate Turabian's A Manual for Writers of Term Papers, Theses, and Dissertations provides a simplified guide to using Chicago style for students.